In the electronics industry, electroless plating and immersion plating are widely used processes for applying metal coatings to substrates. The processes are used extensively, for example, in printed circuit board and integrated package interposer fabrication, and in plating solderable or wirebondable finishes such as nickel-gold to copper surface pads. Electroless and immersion plating have been used to improve bond-pad properties for packaging where the integrated circuit interconnect metal is unacceptable.
Aluminum, for example, is not solderable, although an electroless nickel and immersion gold under-bump metallurgy (UBM) on aluminum bond pads provides solderability for flip-chip assemblies. Aluminum wirebonds well, yet aluminum bond pads on integrated circuits are susceptible to corrosion under standard environmental test conditions. This corrosion can cause performance degradation and product failure when the joint between the gold wire and aluminum pad degrades and fails.
A typical electroless nickel-gold (Ni—Au) bumping process for a flip-chip assembly with conventional aluminum bond pads requires removal of an insulating aluminum oxide layer from the bond-pad surface and good electrical connection to the underlying metal. In the Ni—Au process, oxide removal and surface activation are commonly done through zinc-displacement plating, using a zincate solution. The bump is then formed by selective electroless plating of nickel in a wet chemical, maskless process.
A method for forming a plated nickel layer and a plated gold layer on aluminum electrodes or solder bumps on a semiconductor device is disclosed by Takase et al. in “Method of Forming Electric Pad of Semiconductor Device and Method of Forming Solder Bump”, U.S. Pat. No. 6,028,011 issued Feb. 22, 2000. The gold is applied over a nickel-phosphorous or palladium layer with an electroless gold-plating method in a substitution reaction. Takase does not teach plating a continuous palladium layer, but rather teaches seeding the reaction with palladium. The method avoids corrosion of the passivation film and the aluminum electrodes. Some manufacturers are trying to implement electroless nickel and immersion gold-plated aluminum pads on integrated circuits to provide superior corrosion resistance without encapsulation. The applications may include under-bump metal for flip-chip assembly and wire-bondable finishes for copper pads. These methods for gold and nickel-phosphorous, however, run the risk, like other processes using multiple layers, of creating a mushrooming effect over passivation layers and consequential shorting between pads in fine-pitch arrays. Difficulties in such a process still include creating a uniform bonding surface with no nodules or foreign matter.
In recent years, copper has begun replacing aluminum in some silicon integrated circuits because of its lower resistance, which is good for high-frequency and radio-frequency (RF) circuits. Copper, however, is susceptible to corrosion, and is not easily soldered or wire-bonded. Electroless coatings for copper bond pads are being developed to help solve these problems. One method for processing a wafer having copper bond pads is disclosed by Molla et al. in “Method for Processing a Semiconductor Substrate having a Copper Surface Disposed Thereon and Structure Formed”, U.S. Pat. No. 6,362,089 issued Mar. 26, 2002. The method coats the copper bond pads with solderable or wirebondable metals and allows for the wafer to be processed at temperatures less than or about 90 degrees centigrade. Void-free metal coatings are achieved using a dual activation process: the copper bond pads are activated by placing them in a palladium bath and then in a nickel-boron bath. After the dual activation process, the copper bond pads are coated with a layer of nickel-phosphorous or palladium. The elimination of copper voids enables the use of thin copper interconnects for dense integrated circuits, and increases bond shear strength and bond pull strength between the copper and nickel interface. The Molla invention uses an electroless nickel bath having a low concentration of lead.
Another proposed solution to protecting metal bond pads of conventionally packaged, non-hermetic chip-on-board assemblies is to encapsulate the bonded die with a silicone compound, which helps isolate the pads from standard environmental conditions such as high humidity. Unfortunately, dispensing and curing the silicone is a time-consuming process. The silicone, having a higher dielectric constant and loss tangent than air, may cause a degradation of high-frequency and RF performance. In addition, it is difficult to remove the silicone compound completely, precluding rework and repair.
A partial solution for providing corrosion-resistive bond pads in RF circuits and assemblies is to forego silicone encapsulation on die that are manufactured with gold bond pads, while selectively encapsulating any die in the assembly with aluminum pads. It is known in the art that a gold-to-gold junction is robust with respect to enduring environmental stress such as humidity testing at 85 degrees centigrade and 85 percent relative humidity. This partial solution, however, is limited because many die in the RF section and most of the die in the digital section of an assembly have aluminum bond pads, and therefore must still be encapsulated with the silicone compound for corrosion resistance.
There continues to be a need for a method of making metallic bond pads that are corrosion-resistant and wire-bondable, and do not require encapsulation material such as silicone. The elimination of encapsulation would result in circuits and assemblies that do not require a complex cleaning process, have repairable and reworkable bond pads, have improved reliability of electrical connections to the integrated circuits, and do not have degraded RF performance.
Plated metals on top of aluminum or copper bond pads such as electroless nickel and gold have been shown to have desirable bonding properties and environmental resistance, even without silicone encapsulation. However, high-quality, repeatable electroless plating of integrated circuits is prevented by a number of problems. These problems cause some pads on an integrated circuit or semiconductor wafer to plate differently than others. Plated metals need to be uniformly applied with consistent morphology on all the bond pads of each integrated circuit. This uniformity and consistency is difficult to obtain when bond pads are of different sizes, different locations, and different metallurgies, and have different connectivities to the underlying semiconductor substrate, which result in small yet impactive electric potentials that vary from pad to pad.
Other issues may cause plating problems, such as varying pad metal composition, bond pad porosity, grain boundaries, variations in the surface finish of the pad metal, and thin pad metal. Other potential issues include inadequate passivation coverage, nodule formation, poor plating on pads grounded to the silicon substrate, galvanic cell effects that interrupt the plating process, photovoltaic effects that produce voltages at p-n junctions affecting the plating on associated bond pads, loss of plated material, inadequate coverage, local inhibition of the plating process, and overly aggressive cleaning cycles.
It would be beneficial, therefore, to provide an improved method for uniformly and reliably plating an integrated circuit with corrosion-resistant and wire-bondable metals at the wafer or die level, with the above-mentioned improvements, overcoming the deficiencies and obstacles described above.